The present invention relates to a coil assembly for use in a communication system. More particularly it relates to a coil assembly for use in a communication system in which the spacial orientation of the coil assembly relative to other components in the system can not be predetermined.
There exist numerous communication systems in which communication is to be established between two or more components by means of a linking magnetic field and in which at least one of the components is movable relative to another such that isotropic sensitivity is important at least in a given plane for maintaining communication. The need for isotropic response in paging systems and article surveillance systems, to name two examples, should be readily apparent.
Assuming that communication is to be established either to or from a loop coil by means of an AC magnetic field, i.e., an alternating magnetic field, the problem exists of ensuring adequate magnetic coupling between the coil and the field regardless of the spacial orientation of the coil relative to the lines of flux constituting the field. It is well known, for example, that a flat coil immersed in a magnetic field, wherein all of the lines of flux are parallel to the plane of the coil, will experience little or no magnetic coupling with such field. On the other hand, if the coil is used to produce the field, the lines of flux will be radiated normal to the general plane of the coil and little or no signal will radiate parallel to the coil plane. The action of such coil is clearly anisotropic and null conditions will exist in any communication system in which the relative spacial orientation of the coil can not be predetermined.
In the copending application of Raymond L. Barrett, Jr., entitled "Randomized Tag To Portal Communication System" filed on Mar. 5, 1982, accorded Ser. No. 354,156, now U.S. Pat. No. 4,471,345, and assigned to the same assignee as the present application, there is disclosed a system in which a doorway is provided with a loop coil for establishing an AC magnetic field that is intended to couple with a smaller loop coil carried by a personnel identification tag or marker. In particular, said application describes by way of an example a system for tracking the location of doctors within a hospital facility. It should be appreciated that in any system involving the use of a tag carried by an individual a general constraint may be imposed such that the tag is always carried in a vertical or near vertical orientation. Because of such constraint, the requirement for isotropic tag response to the interrogating field is important only with respect to a plane that is normal to the general plane of the tag.
It is, therefore, an object of the present invention to provide a flat coil that can be used in an identification tag or the like that will have a substantially isotropic response to an interrogating AC magnetic field at least in a plane normal to the general plane of the coil.